Oxide define regions (ODR's) are used in the fabrication of some types of semiconductor devices, such as static random-access memories (SRAM's). Typically, holes are etched in accordance with the desired ODR, so that impurities may be injected, and so on. The ODR's are typically defined by a mask transferred to semiconductor wafers using photoresist. The photoresist protects the oxide from being etched. Where no photoresist is present, the ODR's are defined.
FIGS. 1A-1G illustratively show the typical process for defining ODR's. In FIG. 1A, the semiconductor device 100 being fabricated includes a substrate 102, such as a p substrate. Pad oxide 104 has been grown on the peaks of the substrate 102. The pad oxide 104 may be silicon oxide, SiO2. Silicon nitride 106, Si3N4, is then deposited on the pad oxide 104. High-density plasma (HDP) oxide 108 is deposited over the entire device 100. Peaks in the HDP oxide 108 correspond to the pad oxide 104 and silicon nitride 106 formations below. In FIG. 1B, the HDP oxide 108 is coated with photoresist 110.
Next, in FIG. 1C, a photolithographic process (exposure plus development) is used with a photomask to define the ODR profile having the trench 112, and then the ODR is etched through to the HDP oxide 108, such that the trench 112 exposes the HDP 108 oxide below. In FIG. 1D, the HDP oxide 108 exposed through the trench 112 is itself etched down to the silicon nitride 106, which acts as a stop layer for the etchant. The photoresist 110 is removed in FIG. 1E. In FIG. 1F, a chemical-mechanical planarization (CMP) process is used to plane the HDP oxide 108 down to the silicon nitride 106. Finally, in FIG. 1G, the silicon nitride 106 is etched away.
The process described in conjunction with FIGS. 1A-1G is problematic, however. First, the mask used in FIG. 1C to define the ODR profile cannot precisely open trenches over all the peaks of the HDP oxide 108. The definition of the ODR profile is substantially accomplished so that there is less of the HDP oxide 108 to be processed by CMP in FIG. 1F. Because more of the HDP oxide 108 remains than desired, this lessens the processing window of the CMP, making the CMP more difficult to accomplish. As a result, CMP may have to be redone a number of times in order to accomplish the desired planarization, and in any case more non-uniformity may result. Furthermore, using a mask to define the ODR is costly, inasmuch as a mask must be fabricated, and so on.
Therefore, there is a need to overcome these disadvantages associated with the prior art. Specifically, there is a need to increase the processing window of the CMP when planarizing the HDP oxide 108. There is a need to accomplish such CMP without having to redo the CMP a number of times. For these and other reasons, therefore, there is a need for the present invention.